Spong, U.S. Pat. No. 4,097,895 issued June 27, 1978 and incorporated herein by reference, has disclosed an ablative optical recording medium for use in an optical recording system. The medium comprises a light reflective material which is coated with a layer of a light absorptive organic material. The thickness of the light absorptive layer is chosen so that the reflectivity of the recording medium is reduced. A focused, modulated light beam, such as a light beam from an argon ion laser, when directed at the recording medium vaporizes or ablates the light absorptive layer leaving an opening in this layer and exposing the light reflecting material. Bell, U.S. Pat. No. 4,216,501, which is incorporated herein by reference has disclosed an improved ablative trilayer optical recording medium for use in the Spong optical recording system. The trilayer optical recording medium comprises a light reflective layer, a light transmissive layer overlying the light reflective layer, and a light absorptive layer overlying the light transmissive layer. The thickness of the light absorptive layer is so related to the thickness of the light transmissive layer and the optical constants of the light reflective, transmissive and absorptive layers so as to reduce the optical reflectivity of the recording medium. A focused, modulated light beam ablates or melts the light absorptive layer thus exposing the underlying light reflective layer through the light transmissive layer.
The reflectivity in the area of the opening in the light absorptive layer is essentially that of a light reflective layer and is much greater than that of the surrounding, unexposed region. During readout this difference in reflectivity is detected optically and converted into an electrical signal representative of the recorded information.
To eliminate or reduce signal defects or dropouts caused by surface dust which precipitates onto the medium from the environment, a thick overcoat is applied to the light absorptive layer. Dust particles and other surface contaminants which settle on the upper surface of the overcoat layer are thus far removed from the focal plane of the recording lens so that their effect on the recording or playback signal is considerably reduced. Bloom et al, in U.S. Pat. No. 4,315,269, incorporated herein by reference, disclose a thick overcoat with a preferred range of thicknesses from about 0.05 mm to about 1 mm and describe a thick overcoat about 0.08 mm thick formed by spinning techniques. Bell et al, U.S. Pat. No. 4,101,907 issued July 18, 1978 and incorporated herein by reference, disclose an overcoat structure consisting of a thin layer which forms a chemical and thermal barrier between the light absorbing layer and a thick overcoat layer overlying the thin overcoat. The thin overcoat is typically 0.0003 mm thick and is typically formed by evaporation of silicon dioxide. The thick overcoat is typically about 0.1 mm thick and is formed by spinning techniques.
As the thickness of the overcoat layer is increased it becomes more difficult to obtain an overcoat layer by spinning techniques which has a radially uniform thickness because of the increased viscosity of the overcoat material and the slower spinning speeds required. The optimal thickness of the overcoat layer is a balance between the maximum thickness to provide maximum immunity to surface contamination and a minimum thickness to provide a uniform thickness, in order to reduce optical thickness variations, and to minimize manufacturing costs and time. Overcoat layers about 0.18 mm thick are useful since this thickness corresponds to the standard cover glass correction built into commercially available microscope objectives. Thus, it would be desirable to have an alternative method to spinning for forming thick overcoat layers which are uniform in their thickness.